1. Field of the Invention
The present invention relates to an optical pickup for use in a record/playback device for radiating a light beam to an optical storage medium such as an optical disc to record or play back information.
2. Description of the Related Art
In recent times, high-recording density, large-capacity information-recording media known as DVDs (digital versatile discs) and record/playback systems using such DVDs have become widely known. DVDs are available as DVD-ROMs, DVD-RAMs, DVD-RWs, DVD-Rs, and so on. Although belonging to the same family, DVD-ROMs, DVD-RAMs, DVD-RWs and DVD-Rs are considerably different in optical disc structure. For example, as shown in FIG. 1 of the accompanying drawings, the ROM optical disc structure of the DVD-ROM has a train of embossed pits, which forms a series of concave and convex on the recording surface as recorded information on the recording surface. A series of concave and convex defines a track. The pits are sometimes referred to as information pits. As shown in FIG. 2 of the accompanying drawings, on the other hand, DVD-RW and DVD-R have a groove recordation type optical disc structure in which grooves are formed in the recording surface, which is made from a dye or a phase-change material, and a train of pits possessing different reflectance is formed on the top of the grooves as recorded information. Still another optical disc structure is known, which is called a land-groove recordation type optical disc structure as shown in FIG. 3 of the accompanying drawings. In the land-groove type structure, lands and grooves are formed from the dye or phase-change material, and trains of pits are formed on the lands and grooves. Accordingly, the record/playback systems for the DVD family is required to have compatibility with the different types of optical disc structure.
A three-beam pickup utilizing a three-beam method has been known as a device for tracking the light beam. Three beams are used in the three-beam method to detect the tracking. Specifically, a main beam is radiated onto the recording track to detect information pits on the recording track, and auxiliary beams are radiated on opposite sides of the main beam. For example, three beams (main and auxiliary beams) are emitted to a disc, three optical detectors are provided for the three optical spots of the three returning light beams respectively, and their radial push-pull signals are detected to obtain a differential signal between the radial push-pull signals. This is a differential push-pull method (referred to as “DPP method”) demonstrated with the three-beam pickup, which can realize the tracking error detection substantially independent of the offset. In general, the three light beams are prepared with a diffraction grating. By diffracting a single light beam by the diffraction grating, the zero order diffracted light is taken as the generated main beam and ±first order diffracted light are taken as the auxiliary beams. The three-beam pick can also be used to suppress crosstalk between adjacent tracks. Referring to FIG. 4 of the accompanying drawings, the main beam or the zero order diffracted light is focused onto the target track, and the auxiliary beams or the ±first order diffracted light are focused on the adjacent tracks. Signals are simultaneously read from the adjacent tracks, and these signals are subtracted from a signal originating from the central main beam to cancel the crosstalk. This is a crosstalk canceling (referred to as “CTC”) method accomplished with the three-beam pickup.
When a groove recordation type optical disc is recorded or played back with the three-beam pickup, it is necessary to focus the auxiliary beams between grooves in order to detect the tracking error signal with the DPP method. On the other hand, in order to implement the CTC method on a groove recordation type optical disc with the three-beam pickup, it is necessary to focus the auxiliary beams onto adjacent grooves. In order to ensure compatibility between the operations of the DPP and CTC methods with the three-beam pickup, therefore it is required to vary the focal points of the auxiliary beams by mechanically rotating the grating or varying the pitch of the diffraction element using a liquid crystal element grating.
However, the mechanical grating rotation cannot be momentarily switched to the diffraction element pitch variation, and vice versa. For example, if the auxiliary beams are used for the DPP method during recording and the three-beam pickup wants to perform a playback operation with the CTC method, the change from the DPP method to the CTC method cannot take place instantly. This is unsuited for a high transfer-rate optical disc system that needs to switch between recording and playback very quickly.